1. Field of the Invention
The invention relates to an electric resistance heating unit, and more particularly, to a heating unit having an expanded metal, foil-like strip, as the resistance element, supported on edge along its entire length on an insulating supporting board.
2. Description of the Prior Art
The prior use of a very thin, foil-like resistance heating element, formed of expanded metal and arranged in a grid of a series of parallel corrugations mounted skewer-like on an insulated rod passing through the corrugations, is described in U.S. Pat. No. 3,651,304 granted to Robert J. Fedor on Mar. 21, 1972. Maake, U.S. Pat. No. 3,798,419, is also of interest. The patented Fedor unit offers the advantages of a heating element having high surface area and low mass with attendant savings in raw material, improved heating efficiency, and prevention of sagging which would result in failure of the element.
It has been found that the patented construction involves considerable high cost, hand labor during assembly, permits variation of the spacing between corrugations on the skewer-like support rod under high temperature expansion, yields danger of overheating and even shorting of the strip element, and that the use of support rods limits the flexibility of configurations possible for the heating element.
Various improvements in the above-described patented device are known. Mainly, these involve stringing the expanded metal elements around spaced and fixed ceramic insulators, or passing one edge of the expended-metal strip loosely through grooves in spaced insulators while still retaining the corrugated wavy pattern of the heating element. These improvements still evidence certain disadvantages. For example, manual assembly is required and during assembly a worker must preliminarily string the heating element with the correct mechanical tension to compensate for later thermal expansion so as to prevent sagging at operating temperatures. This requires a high degree of skill and judgment. Also, the ceramic insulators used in conjunction with the heating element must be manually loaded in a metal base, or support frame. Further, a number of desirable patterns or configurations of the heating element are unobtainable, so that the flexibility of patterns in use is restricted.